Get DRM connector reference count while scheduling a prop work
to avoid any possible destroy of DRM connector when it is in
DRM_CONNECTOR_REGISTERED state.

Fixes: a6597faa ("drm/i915: Protect workers against disappearing connectors")
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Tested-by: Karthik B S <karthik.b.s@intel.com>
Signed-off-by: Anshuman Gupta <anshuman.gupta@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111081120.28417-3-anshuman.gupta@intel.com
(cherry picked from commit b3c6661a)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>